Composite structure in aircraft typically utilize a tough skin surface supported by a lightweight core material. Development efforts to increase the strength/weight ratio of the core have resulted in cellular plastic structures such as rigid expanded foam of random cell pattern. Superior structural properties have been realized in cores formed in a geometric honeycomb pattern of hexagonal ducts, achieving very light weight due to the high percentage of air volume: 90% to 98%. Such a core, when sandwiched between two skins, forms a directional structure possessing a uniform crushing strength under compression.
In known art, such cellular or ducted cores are commonly made from thermosetting resins, which are plastics which solidify when first heated under pressure, and which cannot be remelted or remolded, as opposed to thermoplastic resins, which are materials with linear macromolecular structure that will repeatedly soften when heated and harden when cooled.
As utilization of such structures is expanded to include areas previously avoided due to structural demands and temperature, vibration and impact loading environments, new composite matrices are required. Thermosetting resins, commonly used, in most cases, lack the toughness and stability needed for these applications.
New thermoplastic materials offer improved properties; for example composite skin-surfaced structures having honeycomb cores made from preimpregnated thermoplastic fiber material provide excellent impact strength and damage tolerance. However, by their nature, the new thermoplastic materials require new and unconventional processing methods. As opposed to conventional thermosetting processes where sticky and viscous fluids are saturated into reinforcing fiber forms to be cured by catalysis and heat, thermoplastics which have no cure cycle are hard and "boardy" initially, and have to be melted at high temperatures to be worked to the desired shapes. Thus completely different processing schemes are required for thermoplastics than those that have been developed for thermosets. This also holds true for the honeycomb core which is used to give light composite aircraft parts large moments of inertia to multiply stiffness and strength without proportional increases in weight.
In known art, thermoset honeycomb material is made by a process that takes advantage of the flexibility of the reinforcing fabric before it is impregnated with resin. It is bonded and then expanded into hexagon honeycomb structure while it is soft, then wash coated with resin which is subsequently cured to give it its stiffness.
In contrast, thermoplastics, utilized in the present invention for their superior ultimate properties, have no soft stage, and they are too viscous to be wash coated or by some other means saturated into the fabric after bonding the sheets together. The practical options for bonding thermoplastic core material together are further limited by the difficulty of making good adhesive bonds with thermoplastics. For these reasons, thermal fusion bonding of thermoplastic fiber material into a ducted honeycomb structural pattern has been selected as the method for producing strong lightweight core material in the present invention, which addresses new processing methods for realizing the full benefits of the superior ultimate properties of such structure.